In man, suppressor lymphocytes which modulate the response to transplantation antigens in vitro, have been identified in the mixed lymphocyte culture reaction (MLR), the closest in vitro correlate of an in vitro graft-versus-host reaction. Isolation and characterization of this cellular suppressor system appears necessary to understand the mechanisms involved in the control of the immune response to transplantation antigens. Cyclosporin A, an immunosuppressive drug which inhibits cytolytic lymphocyte induction while permitting full activation of suppressor lymphocytes in MLR, will be used to facilitate separation of suppressor and cytotoxic cells which co-purify by most methods. Physical isolation and characterization of human suppressor lymphocytes activated in MLR will be accomplished by velocity sedimentation and density equilibrium centrifugation techniques. We also plan to identify certain membrane characteristics of this regulatory lymphoid cell population such as the receptors for histamine and the Fc portion of the immunoglobulins IgM and/or IgG as well as the ability to adhere to nylon wool. Furthermore, immuno-adsorption techniques will be utilized in an attempt to separate alloantigen-specific and non-specific suppressor lymphocytes. By means of an insolubilized Cyclosporin A column, we plan to separate and identify alloantigen-activated precursor suppressor and cytotoxic cells from MLR. These studies will lay the groundwork for further characterization of suppressor lymphocytes with respect to mode and mechanism of action.